Lake Titicaca study sheds new light on
global climate change

Tropical South America has endured alternating
periods of heavy rainfall and severe drought
during the last 25,000 years, according a new
study in the journal Science.

The report -- based on geological evidence
from one of South America's largest lakes --
demonstrates how nature can produce sudden,
unexpected climate changes that affect the entire
planet.

The study, which appears in the Jan. 26 issue
of Science, uses sediment samples taken
from the bottom of Lake Titicaca -- the world's
highest lake navigable to large vessels.

Straddling the border between Bolivia and
Peru, Titicaca is 120 miles long, 50 miles wide
and has average depth of 500 feet. The lake is
located more than 2 miles above sea level on the
Altiplano, or High Plateau, of the northern Andes
Mountains.

"The Altiplano is like a giant cup, and
Titicaca is the deepest point in the vast
plateau, so most of the precipitation in the
Altiplano drains into the lake," says
Stanford geologist Robert B. Dunbar, one of the
authors of the Science study.

Because very little water drains out of
Titicaca, the lake serves as a reliable archive
of rainfall patterns over many centuries -- not
just on the Altiplano, but in a large portion of
tropical South America, according to Dunbar and
his co-authors.

"Titicaca is the only large and deep
freshwater lake in South America, and in deeper
portions of the lake, sediment has accumulated
continuously for at least the past 25,000
years," they add.

The authors point out that earlier studies of
Titicaca relied on coring samples from the lake
bottom taken at depths of 150 feet or less. To
obtain an older and more complete climate record,
a team of geologists led by Science
co-author Paul A. Baker of Duke University
collected three new samples at 270 feet, 450 feet
and 690 feet below the surface.

Baker, Dunbar and their colleagues were able
to reconstruct a history of precipitation in the
Altiplano by determining how water levels in Lake
Titicaca changed during the last 25,000 years.
The researchers used a variety of techniques to
analyze the salinity, chemistry and microfossil
content of the ancient lakebed.

The most direct method involved counting
fossilized diatoms -- microscopic single-celled
algae often found in lakes. Some diatom species
live near the surface, while others inhabit the
deep. By comparing the abundance of deep- versus
shallow-water fossils in each core sample,
researchers were able to determine whether the
lake level was high or low in a particular
season.

Dramatic changes

After analyzing all three core samples, the
scientists concluded that the lake ­ and
therefore the entire Altiplano -- has undergone a
series of dramatic changes since the Ice Age was
at its peak between 26,000 and 15,000 years ago.

"Lake Titicaca was a deep, fresh and
continuously overflowing lake during the last
glacial stage," according to the Science
study, "signifying that the Altiplano of
Bolivia and Peru and much of the Amazon basin
were wetter than today."

Then, about 15,000 years ago, the Altiplano
underwent a significant change. A dry era was
launched, which continued for the next 2,000
years, causing Lake Titicaca to drop
significantly.

Between 13,000 and 11,500 years ago, Titicaca
began overflowing once again. This wet period was
followed by 1,500 years of relative dryness,
followed by another 2,500 years of heavy
precipitation as the lake again rose to overflow
levels.

Then, about 8,500 years ago, the lake level
fell sharply as the Altiplano again became dry.
But heavy precipitation would return for another
1,000 years, only to be followed by an extremely
dry period between 6,000 and 5,000 years ago,
during which Titicaca fell some 250 feet below
its present-day level ­ its lowest level in
25,000 years.

Titicaca finally began rising again 4,500
years ago. Since then, the southern portion of
the lake has overflowed its banks numerous times.

Millennial time-scale

What caused these thousand-year cycles of
extreme wetness and aridity?

For an answer, the authors turned to
geological climate studies of the Atlantic Ocean.
It turns out that, since the last Ice Age, the
North Atlantic has experienced periods of
unusually cold surface temperature, often lasting
1,000 years or more and accompanied by centuries
of intense precipitation.

According to the authors, these periods of
plunging sea temperatures match the cycles of
extreme wetness revealed in the Lake Titicaca
core samples.

The fact that alternating periods of dryness
and wetness occur on a millennial time-scale or
longer may be influenced, in part, by the
behavior of the Earth as it orbits the sun.

For example, the Earth's rotational axis
gradually changes direction every 26,000 years --
a process called precession. As a result, parts
of the Earth that are relatively close to the sun
during summer today will be farther away during
summer thousands of years from now.

So far, scientists do not have a complete
explanation for the periodic climate changes in
the Altiplano. For example, why did the water
level of Lake Titicaca suddenly plunge to its
lowest level 6,000 years ago?

"This drop occurred very suddenly in just
two or three centuries," notes Dunbar,
"suggesting that there can be rapid changes
that occur in nature that we don't know much
about. Natural variability can be enormous, so
we'd better get a full understanding of how these
systems work before we try to tease out the
impact of humans on climate change."

In addition to Dunbar and Baker, other
co-authors of the Science study are Harold
D. Rowe of Stanford; Geoffrey O. Seltzer of
Syracuse University; Sherilyn C. Fritz and Pedro
M. Tapia of the University of Nebraska; Matthew
J. Grove of Duke University; and James P. Broda
of the Woods Hole Oceanographic Institution.